Finding the next Earth: The Ars guide to exoplanets

Have a faster-than-light colony ship? This is what you should know.

Are we alone in the Universe? For years, people have been making predictions, many using the Drake equation. That involves the use of various educated guesses about the frequency of planets, how many are habitable, and so on. Until about a decade ago, most of the values in the equation remained just that, however: guesses.

In the last dozen years, we've witnessed an amazing transformation in science and appear to be on the verge of several more. The existence of planets orbiting other stars—exoplanets—has gone from a hypothetical to a reality. We've now got a catalog of thousands of potential planets. In many cases, we even have an idea about their size, composition, and temperature. Some of them orbit stars that are, in galactic terms, right next door.

The result has been an incredible buzz of information—over the course of this winter, there were a series of updated estimates on the number of planets in the galaxy (answer: lots) along with various ways of slicing and dicing the numbers. How many Earth-like planets? How many orbiting stars like our Sun? In every case, the numbers were staggeringly large, with the possibility Earth could be one of millions, if not billions, of similar planets in our galaxy alone.

Given all this information, it seems like we're on the verge of finding Earth's twin—a small, rocky, planet sitting at just the right distance from a star to play host to liquid water. But that poses a far more significant challenge than what might be apparent from the field's most recent successes. An understanding of the challenges involved suggests a different time frame, one where we might still be decades away from getting a clear picture of what our galaxy's planets look like.

Spotting exoplanets

There are two main methods we've used for identifying exoplanets: radial velocity and transit. A number of other planets have been spotted through other means, but these two account for the vast majority of sightings.

Radial velocity relies on the fact that gravitational attraction is a two-way street. A star may exert a massive pull on the planets that orbit it but, on a smaller level, those planets pull back. As they swing through their orbits, their gravitational pull tugs the star ever so slightly in the direction of the planet. This motion is enough to cause a slight Doppler shift, the compression or expansion of wavelengths caused as objects move towards or away from an observer. As a star is pulled towards Earth, all of its light gets ever slightly more blue; as it is pulled away, it gets a bit redder.

To track these changes, telescopes have to be fitted with a special instrument called a spectrometer, which tracks how much light is emitted at specific wavelengths. And you have to observe a specific star for an extended period of time to make sure you can spot an acceleration that won't even be visible during portions of its orbit. There are also long- and short-term variations in a star's output (one example is the equivalent of sunspots) that make identifying planets a challenge.

Still, the method is highly effective, and it was the first used to identify an exoplanet. It's especially good if the planet is massive or close to the host star (or both). As a result, many of the first planets we found were what are called "hot Jupiters," gas giants that orbit close in to their host star.

One nice aspect of the radial velocity method is that it provides some indication of the planet's mass, since that determines the strength of its pull on the star. It actually provides a lower limit, since the magnitude of the Doppler effect will be largest if the planet's orbital plane lines up with the line of sight from Earth. You could get the same effect with a heavier planet that has an orbital plane tilted away from Earth.

The alternative is the transit method, which watches for signs of a planet passing between its host star and the Earth. This creates a tell-tale dip in the output from the star, as the planet creates a mini-eclipse. This definitely requires the planet's orbital plane to be lined up so that it runs through Earth. Instead of mass, this method provides an indication of the planet's size, since that determines how much light it can block out.

Combining the two methods gets you both size and mass, which lets you calculate the density of the planet. In multi-planet systems, you can also get this from the transit alone, as the gravitational interactions among the planets can slightly change the timing and duration of the transits.

There are a few other methods used to spot planets, and we've been able to image a number of them directly. But, for the most part, these two account for most of the planets we've observed.

What have we seen?

Initially, there were no instruments designed to detect exoplanets. Instead, researchers had to point existing instruments at a star—and then continue doing so night after night in order to track transits or changes in the velocity of the star. With instrument time at a premium, it's hard to arrange that sort of schedule. So, as a result, the first exoplanets to be spotted were the easiest ones to spot: large, Jupiter class planets orbiting in near their stars. For the first few years of exoplanet hunting, these were the vast majority of the exoplanets spotted.

This raised an obvious question: were we seeing so many just because they were easy to spot, or did this population actually represent the majority of the planets out there? To answer that question, scientists began to design dedicated instruments specifically intended to spot more planets, some ground-based, at least one based in space. In at least one case, a major instrument was fitted with a spectrograph that has specialized in planet hunting. Combined, these instruments began expanding our catalog of exoplanets.

And, in the process, they began to change our picture of our galaxies' planets. Smaller bodies—warm Neptunes and super-Earths—began to appear in the catalog. But we still didn't have a complete enough picture to start making inferences about what the galaxy as a whole looked like.

The Kepler mission was intended to change that, and it has succeeded spectacularly. The space telescope stares down one of the spiral arms of our galaxy, with nearly 150,000 stars in its field of view. Over the past several years, it has found the tell-tale dips in the light that indicates a planet is passing in front of one of them. So far, 105 of these have been confirmed to arise from a planet; there are another 2,740 candidates waiting to be confirmed. We can now start to do statistics.

One of the key things Kepler told us is that most planets are far smaller than Jupiter. First, it became clear there were a lot of Neptune-equivalents, and later, Earth-sized bodies started showing up in the data. It quickly became obvious the numbers went upwards as planet size went down. Rather than being filled with Jupiters, the majority of the planets in our galaxy look much more like Earth. With time, another trend became apparent: the numbers went up the further you got from the star. Not everything was likely to have molten metals bubbling on its surface.

Enlarge/ Even two years ago, Kepler's planetary haul was heavy on Neptune-sized planets and super-Earths. Now, Earth sized planets are the most common things identified.

So, that tells us something about the typical planet. How typical are they? Quite. Detecting a planet using Kepler means the system's orbital plane must be edge-on when viewed from Earth. Given the probability of that happening (which is purely a matter of geometry), we can extrapolate out to how many planets must be in Kepler's field of view. And from that, we can estimate how many planets there are in the galaxy total.

The Milky Way contains about 300 billion stars. On average, each of them has a planet (although many of these are in multi-planet systems, meaning many stars have none). Our galaxy is teeming with planets.

66 Reader Comments

...It tries to account for all of these factors (and a whole lot more), but somehow comes up with the result that the habitable zone around the Sun ends one percent closer to the Sun than Earth's orbit.

Obviously, the Earth seems to be in little danger of having its oceans boil off, which provides some indication this is still an imperfect science. In the same way, the best models have trouble giving Mars the warm and watery past that all the geological evidence points to.

My bold. Given the statement the habitable zone is closer to the sun than the earth, should the water be freezing out rather than boiling off? Or is the statement inverse "... the habitable zone around the Sun ends one percent closer tofurther from the Sun ..." or am I missing something?

I've tried to do a couple of rough calculations to see what it would take to have the ability to get a 1pixel image of an earth sized planet 100LY away.

According to wikipedia:

Quote:

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.The formula takes different forms depending on the units.R = 4.56/D D in inches, R in arcsecondsR = 11.6/D D in centimeters, R in arcsecondswhere D is the diameter of the main lens (aperture), R is the resolving power of the instrument

Diameter of Earth ~12,750km, we'll go with 10,000 as our targetThat means that it has an angular resolution of 10,000km/100LY or roughly 1.05e-11°Multiply by 3600 and we get 3.8e-8 arcsecondsD=11.6/3.8e-8 = 305263158cm, or 3million km !!!!!Coincidently, this happens to be the distance from the L1 to L2 points between the Earth and Sun, and on the far side of the Earth from the Sun. In theory, if it was possible to do inteferometry with two telescopes this far apart, we'd be able to see a planet slightly smaller than earth 100LY away.

Edit: Anyone want to take a crack at whether it'd be possible to resolve <1km at >1,000LY using gravitational lensing around Jupiter or the Sun?

Eyeball Earth would be a kickass place to set a "lost ancient science" based Fantasy novel that's really Sci-Fi in disguise. You'd have the frozen "dark lands" where scary non-human monsters are, as well as many preserved peces of ancient technology. There'd also be ancient technology left over on the light side, some of it solar powered. A paladin/priest class would actually be the remnants of a technological civilization in disguise. There would be a long standing quest to find a lost piece of technology...

I've tried to do a couple of rough calculations to see what it would take to have the ability to get a 1pixel image of an earth sized planet 100LY away.

According to wikipedia:

Quote:

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.The formula takes different forms depending on the units.R = 4.56/D D in inches, R in arcsecondsR = 11.6/D D in centimeters, R in arcsecondswhere D is the diameter of the main lens (aperture), R is the resolving power of the instrument

Diameter of Earth ~12,750km, we'll go with 10,000 as our targetThat means that it has an angular resolution of 10,000km/100LY or roughly 1.05e-11°Multiply by 3600 and we get 3.8e-8 arcsecondsD=11.6/3.8e-8 = 305263158cm, or 3million km !!!!!Coincidently, this happens to be the distance from the L1 to L2 points between the Earth and Sun, and on the far side of the Earth from the Sun. In theory, if it was possible to do inteferometry with two telescopes this far apart, we'd be able to see a planet slightly smaller than earth 100LY away.

Edit: Anyone want to take a crack at whether it'd be possible to resolve <1km at >1,000LY using gravitational lensing around Jupiter or the Sun?

I don't think you have to actually resolve the planet to one pixel, just the space between it and its nearest large body (e.g. the star). If you can do that, you can isolate light scattered from the planet's atmosphere from light that originated directly in the star. For a typical planet in the habitable zone, this would mean a resolution on the order of 100 million km, which is much more reasonable. For planets that orbit further away from very hot stars, its probably doable interferometrically.

Eyeball Earth would be a kickass place to set a "lost ancient science" based Fantasy novel that's really Sci-Fi in disguise. You'd have the frozen "dark lands" where scary non-human monsters are, as well as many preserved peces of ancient technology. There'd also be ancient technology left over on the light side, some of it solar powered. A paladin/priest class would actually be the remnants of a technological civilization in disguise. There would be a long standing quest to find a lost piece of technology...

Well, Earth already has Arctic Monsters (Ursus maritimus), so that part isn't too hard. As for the rest, I think the only way liquid-water based life could survive at all is if an intelligent society deployed a mirror system to melt some of the dark-side ice and return it to its God-Ordained Rightful Place within The Light.

Otherwise all Light-side water will eventually transport via atmospheric circulation (or even diffusion) to the Dark, where it will precipitate as snow and remain there for all eternity locked, unless rescued by the Forces of Light.

Depends on other factors as well, like the total volume of water available. Glaciers do creep, and if there is sufficient water, their edge might creep within the Light, and -- Sacre Deux -- melt. Sort of like Antarctica, only more so.

I'm curious about the geometric factors by which one can estimate the total number of planets in the field of view from those whose orbits intersect Earth. Is there sufficient data from multi-planet systems to determine their orbital planes relative to ours, and the galaxy's? Is there a stellar-system bias to ecliptics that parallel the latter?

So, with several years of Kepler data, how are things looking? Pretty good. One recent analysis focused on small, cool stars called M dwarfs, fairly common in the neighborhood of the Sun. Based on the number of planets spotted orbiting these stars by Kepler, two researchers calculate there's a 95 percent chance of finding an Earth-sized planet orbiting in an M dwarf's habitable zone using the transit method. If we don't require it to transit (in other words, have the planet's orbital plane align with Earth), then the 95 percent probability zone drops to under 25 light years.

I think you're missing a distance number in the first half of this paragraph.

Eyeball Earth would be a kickass place to set a "lost ancient science" based Fantasy novel that's really Sci-Fi in disguise. You'd have the frozen "dark lands" where scary non-human monsters are, as well as many preserved peces of ancient technology. There'd also be ancient technology left over on the light side, some of it solar powered. A paladin/priest class would actually be the remnants of a technological civilization in disguise. There would be a long standing quest to find a lost piece of technology...

Well, Earth already has Arctic Monsters (Ursus maritimus), so that part isn't too hard. As for the rest, I think the only way liquid-water based life could survive at all is if an intelligent society deployed a mirror system to melt some of the dark-side ice and return it to its God-Ordained Rightful Place within The Light.

Otherwise all Light-side water will eventually transport via atmospheric circulation (or even diffusion) to the Dark, where it will precipitate as snow and remain there for all eternity locked, unless rescued by the Forces of Light.

That depends on how much water there is. If the place had an Earthlike amount of water, the oceans would end up mostly frozen on the dark side, but this would create enormous glaciers 4 or five miles thick on the dark side and they would continually push out toward the dark side. The equilibrium condition would have a lot of liquid water being formed by glaciers pushed out past the terminator. That would be a region of lakes or seas, at least where there weren't mountains to block the glaciers' progress.

The prevailing wind would be off of the glaciers, further cooling the zone near the terminator. At high altitude, the prevailing wind would be the other way, carrying some water back to the dark side.

I was reading about the potential for life on earth-like planets in close orbit to a red dwarf. The life could potentially have evolved tens of billions of years before life here, and the system be able to support life tens of billions of years (if not more) longer. If our descendants are still around, we will probably be looking to move to such a system before our star starts cooking us (if we haven't cracked interstellar travel already).

What they didn't talk about (and what I wonder about) are several factors that apply in our system and may not apply in a red dwarf based solar system. Would a less massive star be able to clear the inner solar system of debris so life could evolve on an earth-like world? Would it create CMEs? If so, are they more or less frequent? I doubt we're close to answering either questions yet, but it wouldn't surprise me if there are scientists modelling these sorts of scenarios.

I like to believe that we're eventually going to discover various alien life forms with completely different chemistry than ours. Which could also mean that for some life forms, the "habitable zone" may be different than what we would expect.

I like to believe that we're eventually going to discover various alien life forms with completely different chemistry than ours. Which could also mean that for some life forms, the "habitable zone" may be different than what we would expect.

I'm a little skeptical that fundamentally different chemistry, chemistry developed for existence at temperatures dramatically different from Earth's average, is feasible, or at least anywhere close to as common and easy to form as Earth's standard. If it were, I would expect to find life that first evolved on Earth's poles in temperatures 80K lower than the world average, for example, that is dramatically better at living in such an environment. Instead, we find normal life that has evolved complex coping mechanisms and spread to these hostile environments. Was "natively" developed life that originated near the poles with a fundamentally better-adapted chemistry, while normal life formed around the equator at higher temperatures, really outcompeted by poorly-adapted immigrants? Doubtful. And if you want life to form at temperatures even farther removed than Earth's extremes, you're running into a Hoth-like zero-metabolism deep freeze or boiling oceans.

Eyeball Earth would be a kickass place to set a "lost ancient science" based Fantasy novel that's really Sci-Fi in disguise. You'd have the frozen "dark lands" where scary non-human monsters are, as well as many preserved peces of ancient technology. There'd also be ancient technology left over on the light side, some of it solar powered. A paladin/priest class would actually be the remnants of a technological civilization in disguise. There would be a long standing quest to find a lost piece of technology...

Roger Zelazny wrote a sci-fi fantasy novella called "Jack of Shadow". The Earth is tidally locked. There are the Lightsiders and the Darksiders. He has no shadow on the LightSide and none where there is no light on the dark side. He draws his power from shadow in the shadow zone between Light and Dark. Cool book. Its been out of print for awhile.

I was reading about the potential for life on earth-like planets in close orbit to a red dwarf. The life could potentially have evolved tens of billions of years before life here, and the system be able to support life tens of billions of years (if not more) longer. If our descendants are still around, we will probably be looking to move to such a system before our star starts cooking us (if we haven't cracked interstellar travel already).

What they didn't talk about (and what I wonder about) are several factors that apply in our system and may not apply in a red dwarf based solar system. Would a less massive star be able to clear the inner solar system of debris so life could evolve on an earth-like world? Would it create CMEs? If so, are they more or less frequent? I doubt we're close to answering either questions yet, but it wouldn't surprise me if there are scientists modelling these sorts of scenarios.

Solar storm activity on red dwarfs is much higher than on Sun-like stars. IIRC up to ~10% of the stars normal luminosity, and since any planet in the habitable zone would need to be very close to the star they'd get hammered massively.

I've tried to do a couple of rough calculations to see what it would take to have the ability to get a 1pixel image of an earth sized planet 100LY away.

According to wikipedia:

Quote:

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.The formula takes different forms depending on the units.R = 4.56/D D in inches, R in arcsecondsR = 11.6/D D in centimeters, R in arcsecondswhere D is the diameter of the main lens (aperture), R is the resolving power of the instrument

Diameter of Earth ~12,750km, we'll go with 10,000 as our targetThat means that it has an angular resolution of 10,000km/100LY or roughly 1.05e-11°Multiply by 3600 and we get 3.8e-8 arcsecondsD=11.6/3.8e-8 = 305263158cm, or 3million km !!!!!Coincidently, this happens to be the distance from the L1 to L2 points between the Earth and Sun, and on the far side of the Earth from the Sun. In theory, if it was possible to do inteferometry with two telescopes this far apart, we'd be able to see a planet slightly smaller than earth 100LY away.

Edit: Anyone want to take a crack at whether it'd be possible to resolve <1km at >1,000LY using gravitational lensing around Jupiter or the Sun?

This is also an unusual case because the planet was orbiting around 334 AU from the star so it took two years of additional stargazing after discovery to verify that the object was actually an orbiting planet and not a migratory Death Star or something.

I'm going to have to believe that the number of scientists out there have already refined drakes equation to the value of habitable planets number even if there are a higher categorical presence of planets within the 'Goldilocks' zone. Factors like planet rotation, including the axis a planet rotates and an increased chance of life with the presence of a satellite as either a secondary (or even primary) home or in order to shield the planet from meteor strikes. If there isn't the presence of a satellite, than there might still be a sign of life but the chance intelligent life has to reemerge to be interstellar would potentially take longer to develop.

Those who are fond of Stephen Baxter novels might also take into consideration that our alien counterparts could be as smart (or as dumb) as we are. That life is everywhere, but the ability to travel between stars or to communicate between is far more difficult than what we currently believe. Some of these questions will be answered of course when Voyager finally passes through that interstellar barrier. (Questions like is there enough substance between stars/too much vacuum).

I don't think you have to actually resolve the planet to one pixel, just the space between it and its nearest large body (e.g. the star). If you can do that, you can isolate light scattered from the planet's atmosphere from light that originated directly in the star. For a typical planet in the habitable zone, this would mean a resolution on the order of 100 million km, which is much more reasonable. For planets that orbit further away from very hot stars, its probably doable interferometrically.

Yeah, I didn't really think about that. Even that level of resolution would require a baseline of 300km. That would be doable with two telescope on the moon.

Go get it! We'll have two earths! Or maybe we'll open trade negotiations with whoever lives there. Or wave "hello" or send smoke signals or something.

In all seriousness, finding another earth would be a ridiculously huge breakthrough for a myriad of reasons. We'd have a whole 'nother set of data on everything from evolution to geology to climatology to just about every other field of science there is. Whether things on the other planet are very much like ours or very different, it would tell us tons. Did life on that planet evolve similarly to how it evolved here (meaning did it end up with animals similar to bears, horses, cats, dogs, etc.?) If so, that would suggest that the path that evolution has taken on our world is the best/most likely path, rather than just chance. And if there's intelligent life there, we can share information with them: perhaps we've progressed in ways they haven't and they've progressed in ways we haven't. New perspectives on culture, art, music, everything.

Of course, that's assuming we were able to develop a method of traveling there or otherwise gathering all the data I just mentioned, which is another reason it would be incredible: right now long-distance space travel is mostly a novelty idea. I'm sure we'll learn a lot whenever we get around to sending people to Mars, but it's basically a big rock in space, so there's not much incentive beyond our curiosity about exactly what kind of rock it is, which is why we're not still going to the moon every day: it's awesome that we can, but there's just not much to do there and it's crazy expensive to get to it. All of that changes if we find another earth: all of the science that could be done there, mining for resources, and I'm sure there are plenty of other reasons that I can't imagine. And the intelligent life thing would just make it that much more of a big deal. All of that would create the necessity for us to get there and/or build telescopes that can see it. Necessity is the mother of invention, so add the necessity and we've suddenly got a lot more invention going on trying to develop better, faster, less expensive ways of traveling and peeking through space!

I've tried to do a couple of rough calculations to see what it would take to have the ability to get a 1pixel image of an earth sized planet 100LY away.

According to wikipedia:

Quote:

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.The formula takes different forms depending on the units.R = 4.56/D D in inches, R in arcsecondsR = 11.6/D D in centimeters, R in arcsecondswhere D is the diameter of the main lens (aperture), R is the resolving power of the instrument

Diameter of Earth ~12,750km, we'll go with 10,000 as our targetThat means that it has an angular resolution of 10,000km/100LY or roughly 1.05e-11°Multiply by 3600 and we get 3.8e-8 arcsecondsD=11.6/3.8e-8 = 305263158cm, or 3million km !!!!!Coincidently, this happens to be the distance from the L1 to L2 points between the Earth and Sun, and on the far side of the Earth from the Sun. In theory, if it was possible to do inteferometry with two telescopes this far apart, we'd be able to see a planet slightly smaller than earth 100LY away.

Edit: Anyone want to take a crack at whether it'd be possible to resolve <1km at >1,000LY using gravitational lensing around Jupiter or the Sun?

You'r not actually trying to resolve the planet, you just want to resolve the distance between the star and planet. Both objects will appear as point sources on your detector. So, if you want to resolve an Earth-Sun analog at d=100ly (I'm going to turn this into d=30pc for ease of the calculation), the separation in arcsec is 1AU/30pc = 0.033 arcsec. The resolving power of a telescope is directly roughly equal to the wavelength that you're interested in divided by the mirror size: lambda/D. So, say we want to look in the Infrared (the visible is difficult from the ground due to poor correction of the atmospheric turbulence) with lambda=5 microns, then the minimum mirror size to resolve such a separation is :D = (5E-6 meters) x (206265 arcsec/radian) / (0.033 arcsec)D ~ 30 meters

This size coincides nicely with the next-generation extremely large telescopes currently being developed, including the E-ELT (39 meters), TMT (30 meters), and GMT (24.5 meters). Although, we already have a ground-based telescope, the Large Binocular Telescope, which has a resolving power equivalent to a 22.8-meter telescope due to use of interferometry.

However, when we're talking about resolving Earth-like planets around nearby stars, the limiting factor isn't necessarily the resolving power of your telescope, but rather the "contrast limit." In an ideal world, you would have a perfect point-source function (PSF) dependent on your telescope aperture (mirror size). However, atmospheric turbulence refracts the light from your star in random directions, creating a smeared out image. These days, we have adaptive optics systems that use deformable mirrors to quickly correct for the atmospheric distortions and reconstruct the telescope PSF based on a guide star. Unfortunately, these systems aren't perfect, leaving residual 'speckles' that can mimic planetary signals. At this level, we are at the aforementioned contrast limit, which extend out to about 3 times the Rayleigh (Dawes) criterion. Current contrast limits are about 10^-5 times the brightness of the star (and always improving!) in the Infrared within the angular region. For direct imaging of exo-Earths, we need about 10^-10 contrast.

There are also a fair amount of other direct imaging techniques being investigated and developed that use coronographic systems or pupil plane masks to generate interferometric images in order to reconstruct high contrast images from the phase information, but all of them have their shortcomings and nothing will even come close to directly imaging an exo-Earth until the next-gen telescopes start to come online.

I read that as "earth is on the inner most edge of the habitable zone."

Basically, if we were just a mite closer to the sun, our oceans would be boiling.

Also, John, another great piece on both the (quite literally) astronomical successes of science lately, while also addressing it's severe short fallings. Well done, sir.

But that's based on a very crude model. They should be looking to climatologists for refinement of their model.

As a matter of fact, some astronomers that model exoplanet atmospheres are reaching out to meteorologists and climatologists to learn about atmospheric dynamics, feedback effects, etc in order to incorporate relevant dynamics into their exoplanet models rather than reinventing the wheel. For instance, collaborations between the two fields has been very useful in refining 3D hydrodynamic global circulation models for a transiting Hot Jupiter in order to match the observed Infrared photometry during the planet's primary and secondary eclipses.

I like to believe that we're eventually going to discover various alien life forms with completely different chemistry than ours. Which could also mean that for some life forms, the "habitable zone" may be different than what we would expect.

Actually we have already discovered different chemistry than ours. See here:

Also, I would like to thank John for providing a wonderfully accessible article. As you are writing precisely about my field of study, the care and effort in producing this piece is definitely appreciated. The only thing that really wasn't discussed is the future of exoplanet studies. As stated, one of the ultimate goals of this more-or-less emerging field is to detect and characterize an Earth-like planet capable of harboring life as we know it.

Transits are actually exceptionally useful in this regard, especially when it comes to multiple planet systems. For instance, Kepler has been able to use variations in transit timing to derive the masses of the planets. These transit timing variations result from the gravitational pull of all the planets on each other as they orbit the star. This is most apparent in lower-mass M-stars where all the planets are tightly packed at small orbital radii. But basically, this means that we don't always need radial velocity measurements to derive the masses, which is great for the M-stars because the strong photospheric activity makes it difficult to pull out the individual RV signals. So now, Kepler has given us the masses and radii of some planets, which gives us constraints on the bulk composition (rock, ice, gas).

However, this isn't enough to tell us anything about the atmospheric composition. Is there an atmosphere at all? How heavy is it? What are the primary gas components? Models can be useful, but with so many free parameters and unknowns, they will most likely be wrong at this stage of the game. The next step then is to acquire multi-wavelength observations of the transits as the planet orbits. Kepler cannot help us here, because it only has a single broad photometric band in order to integrate as many photons as possible. The next generation telescope needs to disperse this same light across multiple pixels, dramatically reducing the S/N, and still detect changes in the flux at 10-100 ppm. The are a few projects in the works to do exactly that currently under consideration or already starting development: CHEOPS (ESA, Swiss-led), EChO (ESA), and FINESSE (NASA).

While that's great for planets that have multiple transits on short timescales (those around M-stars with close orbits near the habitable zone), Earth-Sun analogs will take a year between transits. Building up the requisite S/N over multiple transits just isn't feasible in this type of scenario, so the alternative is to move to direct imaging experiments where we physically resolve the planet from the star. For this to be feasible, we require large-aperture telescopes, such as the next-generation so-called ELTs (30-meter class telescopes), including the E-ELT, GMT, and TMT, which are slated to start coming online in 2020, while full science operations probably wont commence for a few years after due to the huge engineering requirements.

In addition, the James Webb Space Telescope (JWST) will be a huge boon to direct imaging efforts, because it's a relatively large aperture telescope (6.5-meter mirror) in space that doesn't have to contend with atmospheric turbulence or absorption of those same molecules that we are trying to detect within the atmospheres of exoplanets! While not necessarily part of the formal mission requirements, I have no doubt that JWST will provide constraints on the atmospheric composition of planets around nearby stars. Astronomers are very good at pushing technology to their absolute limits, such as with the very clever transiting observations being done with the Spitzer Space Telescope, a technique never envisioned by the original engineers when this thing was launched 10 years ago.

Sometimes the facts overturned prior major theories about the way things worked on Earth. For example, most of the oxygen created by the Amazon is sucked back in and reused for the decomposition process. Very little, if any is ever used outside the Amazon. To me, this is the best NOVA show that I've ever seen in my entire life after watching NOVA for 20+ yrs. Simply amazing footage, science and explanations.

Multiple "facts" that I was taught in college and prior schools were overturned and expanded with real data to further back up some old and apparently correct theories too.

I like to believe that we're eventually going to discover various alien life forms with completely different chemistry than ours. Which could also mean that for some life forms, the "habitable zone" may be different than what we would expect.

Actually we have already discovered different chemistry than ours. See here:

Thats just an alternative form of autotrophy (different as in different from photosynthesis and more common forms of autolitotrophy). Everything after carbon fixation proceeds pretty much like on every other life form on earth. In fact many organism like us (heterotrophs) get energy/nutrients by digesting chemosynthetic organisms. Its just a variant form of reducing oxidized carbon. All other parts of biochemistry are identical.

Chemosynthetic organisms are no more alien to humans than photosynthetic microbes or plants.

I'm going to have to believe that the number of scientists out there have already refined drakes equation to the value of habitable planets number even if there are a higher categorical presence of planets within the 'Goldilocks' zone. Factors like planet rotation, including the axis a planet rotates and an increased chance of life with the presence of a satellite as either a secondary (or even primary) home or in order to shield the planet from meteor strikes. If there isn't the presence of a satellite, than there might still be a sign of life but the chance intelligent life has to reemerge to be interstellar would potentially take longer to develop.

Those who are fond of Stephen Baxter novels might also take into consideration that our alien counterparts could be as smart (or as dumb) as we are. That life is everywhere, but the ability to travel between stars or to communicate between is far more difficult than what we currently believe. Some of these questions will be answered of course when Voyager finally passes through that interstellar barrier. (Questions like is there enough substance between stars/too much vacuum).

I think the goldilocks-zone is only part of what is required and whether a satellite is necessary to create tidal forces is still unknown (but a fantastic theory when compared to some crap that school boards are pushing in some places). A magnetic field protects the Earth, plants and animals from all sorts of nasty radiation both from the Sun and all the other stars. A weak magnetic field may be why Mars doesn't have life today. http://www.universetoday.com/14949/mars-magnetic-field/

When it comes to space travel, the average person doesn't understand that it would take 4,000+ yrs to travel to the closest star from the Sun assuming instantaneous velocity at the speed of the fastest man-made object ever created and instantaneous stopping at arrival. Without some huge fundamental scientific discovery - well beyond the relativistic physics discovery - humans are not going to other star systems. Not ever. We will die out when the Earth is swallowed by the Sun becoming a red giant star - assuming we aren't killed off by gamma radiation from another star or huge asteroid impacts or stupid human destruction of the planetary ecosystem by any of the current means in place today. There is a time limit for life on Earth and the clock is definitely ticking.

It is my understanding that humans cannot safely travel through space, only robots and machines. If we cannot get there, are we putting the cart before the horse?

Will we ever feasibly be able to travel any significant distance in space? Or is that going to be up to Skynet and it's progeny?

Edit- Out of curiosity, would machines capable of thought and action of their own be considered to have evolved from us humans? I'm curious, because it seems to me there is a whole lot of space waiting to be explored and wouldn't it be a waste if life never could? I mean, except that it is, right now, by machines. But I don't consider those alive yet the same way I don't consider a virus to be alive.